STABILIZED LASER SOURCE WITH VERY HIGH RELATIVE FEEDBACK AND NARROW BANDWIDTH
This invention relates to the stabilization of a laser source used in opto-electronics, specifically a source comprising a semiconductor laser diode. Such laser sources are often used as so-called pump lasers for fiber amplifiers in the field of optical communication, erbium-doped fiber amplifiers being a prominent example. Such lasers are usually designed to provide a narrow-bandwidth optical radiation with a stable power output in a given frequency band. The present invention now concerns such a laser source using external reflector means, preferably consisting of one or more appropriately designed fiber Bragg gratings, providing very high relative feedback with an extremely narrow bandwidth, combined with a very long external cavity encompassing about 100 modes or more and an extremely low front facet reflectivity of the laser diode. Also, the FWHM bandwidth of the external reflector is selected to be significantly smaller than the distance between the modes within the laser diode. This stabilizes the laser source extremely well in its operation, even without an active temperature stabilizing element.
1 . A high power laser source for generating a stable multimode exit beam at
a desired wavelength, said laser source comprising a laser diode and guide means for conducting a laser beam, said laser diode including a low reflectivity front facet and a high reflectivity back facet, and said guide means including at least one external reflector, wherein
said external reflector forms a dominant long cavity with said back facet of said laser diode,
said external reflector has a full-width-half-maximum (FWHM) bandwidth and a peak reflectivity R FBG centered at the desired wavelength of said exit beam,
said laser diode encompasses a number of longitudinal modes whose separation is larger than said FWHM bandwidth of said external reflector,
said FWHM bandwidth of said external reflector is not larger than 80%, preferably lower than 50%, of said mode separation,
said long cavity is of sufficient length to encompass several tens of modes at said desired wavelength,
said front facet has a reflectivity R F smaller than said reflectivity R FBG and
said reflectivities R FBG and R F being selected to achieve a predetermined relative feedback
r FB =π 2 *R FBG /R F ,
η being the coupling efficiency to said guide means.
2 . The laser source according to claim 1 , wherein
the relative feedback r FB is higher than 1, preferably higher than 10.
3 . The laser source according to claim 1 , wherein
the long cavity has a length of at least 0.5 m, preferably about 2 m.
4 . The laser source according to claim 1 , wherein
the distance of the longitudinal modes in the laser diode is at least 30 pm or larger, preferably 35 to 60 pm, whereas the FWHM bandwidth of the external reflector is less than 25 pm, preferably 15 to 20 pm.
5 . The laser source according to claim 1 , wherein
the reflectivity R F of the laser's front facet is equal or less than 0.5%.
6 . The laser source according to claim 1 , wherein
the factor η, the coupling efficiency, is between about 0.5 and 0.9, preferably between about 0.65 and 0.85.
7 . The laser source according to claim 1 , wherein
the laser source is uncooled.
8 . The laser source according to claim 1 , wherein
the guide means includes a waveguide consisting of or comprising silicon nitride (Si 3 N 4 ), silica (SiO 2 ), or silicon (Si).
9 . The laser source according to claim 1 , wherein
the external reflector is a grating, in particular a fiber Bragg grating, integrated within the guide means.
10 . The laser source according to claim 9 , wherein
two or more gratings are provided, at least one of them integrated within the guide means.
11 . The laser source according to claim 9 , wherein
the grating exhibits a non-uniform reflection characteristic resulting in a predetermined filter function, in particular a filter function with a linear shape or a flat-top shape.
12 . The laser source according to claim 9 , wherein
the grating is a chirped grating resulting in a preselected chirped filter function shape.
13 . The laser source according to claim 9 , wherein
the grating is an apodized grating resulting in a filter function with suppressed side-band maxima.
14 . The laser source according to claim 11 , wherein
at least one of the gratings is a chirped and apodized grating resulting in a preselected chirped filter function with suppressed side-band maxima.
15 . The laser source according to claim 1 , wherein
an electronic dither is superimposed on an injection current of the laser diode for improving the power stability of the laser exit beam.
16 . The laser source according to claim 1 , wherein
the laser is a semiconductor diode laser, in particular an InGaAs quantum well diode laser.
17 . The laser source according to claim 1 , wherein
the laser guide means comprises a polarization-maintaining or a non-polarization-maintaining optical fiber.
18 . The laser source according to claim 1 , wherein
the guide means includes means for directing the laser beam into an optical fiber, in particular beam collimating or focusing means attached to or integrated into said optical fiber.
19 . A laser source according to claim 1 for a frequency doubling system, in particular a blue or green laser system.
20 . Use of a laser source according to claim 1 as pump laser for a fiber amplifier, in particular an erbium-doped fiber amplifier.
21 . A fiber amplifier for optical communication purposes, in particular an erbium-doped fiber amplifier, including
a laser source as pump laser according to claim 1 .